Stock Based Compensation (Tables)

3 Months Ended

Mar. 31, 2014

Summary of Option Activities

The following table summarizes option activity under our 2013 Equity Incentive Plan and related information during the three months ended March 31, 2014:

				Shares
				Subject to			Weighted-
	Shares Available			Outstanding			Average Exercise
	for Grant			Options			Price Per Share
Balance at December 31, 2013		81,948			3,708,773		$	9.43
Options authorized		2,045,785			—			—
Options granted		(671,613	)		671,613			25.14
Options exercised		—			(157,833	)		6.39
Options canceled		19,732			(19,732	)		17.76
Balance at March 31, 2014		1,475,852			4,202,821		$	12.02

Fair Values of Employee Stock Options

The estimated grant date fair values of the employee stock options were calculated using the Black Scholes valuation model, based on the following assumptions:

	Three Months Ended March 31,
	2014			2013
Risk-free interest rate		1.88	%		1.10	%
Expected life	6.0 years			6.0 years
Expected Volatility		79	%		72	%
Dividend yield		—			—

Classification of Stock-Based Compensation Expense

	Three Months Ended March 31,
	2014		2013
Research and development	$	1,066	$	336
General and administrative		1,236		377
Total stock-based compensation	$	2,302	$	713